1. Field of the Invention
The present invention relates to the field of integrated circuit testing systems. More particularly, it relates to methods and apparatus for efficiently positioning an integrated circuit chip as a device under test in a multi-pin feed-through interface module/test socket fixture.
2. Background
As integrated circuits (ICs) have become more complicated, means for testing IC chips have similarly become more complicated and more expensive. These chips often have several hundred connector pins which challenge the tester to provide complete and secure electrical contact with each pin, in a non-destructive way. Such chips inherently generate significant amounts of heat which must be dissipated. It is common that only a few prototype chips are fabricated for intensive testing and rework, until satisfactory results are achieved. Accordingly, cost effective chip development requires chip test fixtures that provide for: (1) non-destructive, easy installation and removal of the chip from the test device; (2) general emulation of the chip's normal operating environment in terms of pin connections, power applied, output measurement and heat dissipation; and (3) means to negate the effects of special conditions related to the test device itself such as special power application requirements and related special grounding, special heat dissipation, etc.
One type of known IC testing system is the Schlumberger IDS5OOOPlus e-beam probe station (the "probe station"). Referring to FIG. 1, the probe station uses a scanning electron microscope (SEM) 10 to image the IC or device under test ("DUT") 12 as well as detect variations in the electric potential of the surface of the DUT 12. These surface potentials are then translated into device signal wave forms by the computer controller 14 and displayed on the color monitor 16 in a manner similar to that seen on an oscilloscope. Test engineers viewing these wave forms are thereby able to compare the DUT's actual performance characteristics with corresponding design criteria. In order to test a chip in this type of test system, the chip must be mounted on a DUT interface board which is then attached to a feed-through interface Module 20. The feed-through interface Module 20 is then positioned to form the top of the vacuum chamber 22 in such a way as to place the DUT 12 directly in line with the electron stream generated by the SEM 10. Schlumberger produces a number of other models of such test systems, such as the IDS 3000, IDS 5000, and IDS 5000HX, all of which use the same basic test fixtures.
A type of feed-through interface Module called a Universal Load Module is sold by Schlumberger for use with their probe station. Referring now to FIG. 2, the Schlumberger Universal Load Module 30 forms the top of the vacuum chamber and has 600 spring-loaded `pogo-pins` 31 which make contact to the DUT interface board 34. These pogo-pins 31 provide a signal path from the DUT interface board 34 through the Universal Load Module 30 to the test equipment outside of the vacuum chamber.
The DUT 12 fits into some type of mounting 36 on the DUT interface board 34. This mounting 36 must provide both electrical contact to the DUT pins 38 as well as mechanical support to the DUT 12 as the test unit is moved around.
A particular test system configuration combines a 600-pin Universal Load Module from Schlumberger 30 with a DUT interface board 34 from Fresh Test Technology of Gilbert, Ariz. This particular arrangement is specific to integrated circuits in cavity-down PGA (pin grid array) packages, but may be adapted to other cavity-down package configurations.
Existing mounting socket options which meet the working distance operating parameters of such test devices are of two basic types;
(1) Positive insertion force sockets; and PA1 (2) Zero insertion force sockets (spring sockets).
Positive insertion force PGA sockets provide both electrical contact and mechanical support through the same mechanism. Their basic design is a cylinder which fits around the DUT pin and holds the pin through some flexible mechanism which contacts the sides of the pin. Examples of such sockets are Swart Interconnect model RC-PGA-11-10-084-M4-01L-17 or Hypertac versions such as Swart Interconnect model SI-337-Y4-AH-20-F.
Zero insertion force PGA sockets of the type made by Fresh test Technology, provide electrical contact, but do not provide mechanical support. Their basic design is also a cylinder which fits around the DUT pin, but does not contact it along the sides. Rather, a small spring-loaded contact is located in the bottom of the cylinder which contacts only the bottom of the DUT pin. Thus the device is free to move up or down in the cylinder with `zero force`.
Because of the high pin counts of some PGA IC's, the positive-insertion force socket of the Swart Interconnect type becomes undesirable due to the large forces required to insert and extract the devices from the sockets. For example, a 300 pin IC inserted into positive-insertion force sockets of 1 oz. each would experience over eighteen pounds of force. In the test and analysis scenario, such insertion and extraction can take place on an hourly basis, frequently subjecting the devices to risk of physical damage. Thus the importance of the zero-insertion force socket becomes apparent. However, the nature of zero-insertion force sockets is that the device requires some form of mechanism to compress the DUT pins onto the spring-loaded contacts in the base of the socket and to hold it there as a mechanical support. The mechanism provided by Fresh Test Technology with their spring socketed DUT boards is referred to as a "hold-down" or hold-down plate. Referring again to FIG. 2, the hold-down 40 consists of a stainless steel plate with holes 42 drilled at each corner and four screws 44 with corresponding `PEM` nuts 46 to attach the hold-down 40 to the DUT interface board 34 and compress the pins 38 of the DUT 12 into the spring sockets 36.